In order to support a driver of a motor vehicle in fulfilling his tasks in road traffic, it is commonly known that a vehicle—e.g. a passenger car—may be adapted or configured for presenting extracted sign information within the vehicle and/or adapting one or more vehicle functions based on detected sign information captured by an image capturing device on-board the vehicle from sign information presented on a road sign detectable by the vehicle. In this manner—should the road sign refer to for instance a speed limit sign—an interpreted speed limit value may conveniently be presented on e.g. a display within the vehicle and/or a vehicle speed may conveniently be adapted in consideration of the detected speed limit. A road sign may however further have assigned thereto a supplementary sign, which commonly is represented by e.g. a small rectangular sign beneath the main road sign. A supplementary sign may for instance state “tractor”, thus indicating that the road sign to which the supplementary sign is assigned pertains only to e.g. tractors, and not to e.g. passenger cars. Determining which sign information that pertains to a vehicle and which does not is however challenging, and it is not uncommon that in e.g. a passenger car, an interpreted speed limit value is presented and/or a vehicle speed is adapted in consideration of the detected speed limit, despite of the fact that the speed limit does not pertain to said passenger car. US 2013/0049988 for instance, relates to traffic sign recognition, and discloses how an optical system on-board the vehicle detects the traffic sign and determines information related to the sign, how driving maneuvers carried out by the vehicle are monitored to determine driving situation information, and further how the driving situation information and the traffic sign information are used to determine a probability that the sign is pertinent for the vehicle under the current driving conditions. If the probability is above a threshold, the driver is alerted to the traffic sign. However, although US 2013/0049988 determines whether or not to alert a driver of a vehicle to the presence of a traffic sign in the vehicle's driving environment, the challenge of determining which sign information that pertains to a vehicle and which does not, still remains. For instance, such an image capturing device on-board a vehicle discussed above—e.g. represented by a camera—commonly have detection difficulties in a number of environmental conditions such as in heavy rain or snow, fog or direct sunlight etc., and/or if a road sign is at least partly covered by e.g. dirt and/or positioned at the edge of the camera view. Not in least when it comes to supplementary signs and/or supplementary sign information may said detection difficulties be especially challenging, thus rendering that a vehicle incorrectly may present and/or adapt to road sign information actually non-pertinent to said vehicle.